Each user of my application will choose their country, after which it will be stored in a cookie and stored for later requests. Everything is working OK, but I need to set t
You must set the culture on each call, Session_Start is only fired when a session is created. So your culture is correct only on first call or when the session is recreated.
You are assuming that the thread that will service the page request is the same thread that has started the session as in your code - this is not guaranteed.
You may want to save the culture in a Session
variable and use an override InitializeCulture in your pages, as described in: How to: Set the Culture and UI Culture for ASP.NET Web Page Globalization.